Some transport refrigeration units are of a type that have their compressors driven solely by an electric motor, or at times by an electric motor, as distinct from an internal combustion engine with a liquid cooling system. In many instances heat derived from the engine is applied to the refrigerant accumulator of the system and this heat is then available in a defrost cycle to promote and hasten defrosting. It is known that when a refrigeration system is switched from cooling to defrost, there is initially a very low suction and discharge pressure with correspondingly low mass flow of refrigerant, and the increase in the mass flow is at a very slow rate. With an electric motor driven compressor, there is no heat from an engine and in many cases the defrost cycle with hot gas would be inordinately long unless electric heaters were also used to aid in defrosting.
It would therefore be desirable to provide a system in which additional heat is available to the refrigerant to promote and hasten the defrost cycle, and it is to such a system that this invention is directed.